text: In discussing these figures, it should be kept in mind that both the radon distribution in dwelling and its effect at low exposures are not precisely known, and the radon health effect has to be computed (deaths caused by radon domestic exposure cannot be observed as such). These estimations are strongly dependent on the model retained.
  According to these models, radon exposure is thought to be the second major cause of lung cancer after smoking.
  Iowa has the highest average radon concentration in the United States; studies performed there have demonstrated a 50% increased lung cancer risk with prolonged radon exposure above the EPA's action level of 4 pCi/L.
  Based on studies carried out by the National Academy of Sciences in the United States, radon would thus be the second leading cause of lung cancer after smoking, and accounts for 15,000 to 22,000 cancer deaths per year in the US alone.
  The United States Environmental Protection Agency (EPA) says that radon is the number one cause of lung cancer among non-smokers.
  The general population is exposed to small amounts of polonium as a radon daughter in indoor air; the isotopes Po and Po are thought to cause the majority of the estimated 15,000–22,000 lung cancer deaths in the US every year that have been attributed to indoor radon.
  The Surgeon General of the United States has reported that over 20,000 Americans die each year of radon-related lung cancer.
  In the United Kingdom, residential radon would be, after cigarette smoking, the second most frequent cause of lung cancer deaths: according to models, 83.9% of deaths are attributed to smoking only, 1.0% to radon only, and 5.5% to a combination of radon and smoking.
  The World Health Organization has recommended a radon reference concentration of 100 Bq/m (2.7 pCi/L). The European Union recommends that action should be taken starting from concentrations of 400 Bq/m (11 pCi/L) for older dwellings and 200 Bq/m (5 pCi/L) for newer ones. After publication of the North American and European Pooling Studies, Health Canada proposed a new guideline that lowers their action level from 800 to 200 Bq/m (22 to 5 pCi/L).
  The United States Environmental Protection Agency (EPA) strongly recommends action for any dwelling with a concentration higher than 148 Bq/m (4 pCi/L),
  and encourages action starting at 74 Bq/m (2 pCi/L).
  EPA recommends that all homes should be monitored for radon. If testing shows levels less than 4 picocuries radon per liter of air (160 Bq/m), then no action is necessary. For levels of 20 picocuries radon per liter of air (800 Bq/m) or higher, the home owner should consider some type of procedure to decrease indoor radon levels. For instance, as radon has a half-life of four days, opening the windows once a day can cut the mean radon concentration to one fourth of its level.
  The United States Environmental Protection Agency (EPA) recommends homes be fixed if an occupant's long-term exposure will average 4 picocuries per liter (pCi/L) that is 148 Bq/m. EPA estimates that one in 15 homes in the United States has radon levels above the recommended guideline of 4 pCi/L.
  EPA radon risk level tables including comparisons to other risks encountered in life are available in their citizen's guide.
  The EPA estimates that nationally, 8% to 12% of all dwellings are above their maximum "safe levels" (four picocuries per liter—the equivalent to roughly 200 chest x-rays). The United States Surgeon General and the EPA both recommend that all homes be tested for radon.
  The limits retained do not correspond to a known threshold in the biological effect, but are determined by a cost-efficiency analysis. EPA believes that a 150 Bq/m level (4 pCi/L) is achievable in the majority of homes for a reasonable cost, the average cost per life saved by using this action level is about $700,000.
  For radon concentration in drinkable water, the World Health Organization issued as guidelines (1988) that remedial action should be considered when the radon activity exceeded 100 kBq/m in a building, and remedial action should be considered without long delay if exceeding 400 kBq/m.